Zincs are gone at the end of the season!

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Jun 4, 2004
81
Hunter 28 Boothbay
Are there any electrical engineers out there that can give me directions on how to test for electrolysis? Each year I put on two 'egg' zincs and they are both completely gone when I haul at the end of the year. I am on a mooring on a river in Maine and not near any marinas or sources or AC (that I know of). Could this problem be confined to my boat?
 
Sep 25, 2008
7,077
Alden 50 Sarasota, Florida
easy test

The most common method is to use a "half-silver chloride cell." Simply using a standard voltmeter by itself is useless. The cell consists of a piece of pure silver, connected by a copper wire to a sensitive voltmeter and then with another copper wire to a probe that can be used to make electrical contact with various pieces of underwater metal. The Silver/Silver Chloride (or Ag/AgCl) reference electrode is the reference electrode of choice, because it’s easily and cheaply prepared. Almost any digital voltmeter can be used to take the measurements. Analog voltmeters that can read voltages as low as 1/1000 of a volt (one millivolt, or mv) can also be used, except an analog meter will give you a very low reading (if any at all) in fresh water. To use the voltmeter to check on the adequacy of the sacrificial zinc, one lead wire is connected to the silver electrode and immersed in the water in which the boat is floating. The other wire from the voltmeter is connected to a piece of metal in the boat that is in contact with the seawater (the prop shaft, for example). Voltage Ranges vs Silver/Silver Chloride Reference Electrode Bronze: 500 - 700 mV < 500 mV Bz is eroding - Add zinc, > 700 mV Bz overprotected - Remove zinc Steel: 750 - 950 mV Aluminum: 800 - 1050 mV Lover milli-Voltages indicate metal erosion - ADD Zinc. Higher mill-Voltages indicate over-protection - Remove Zinc. Galvanic Series of Metals in Sea Water from the least noble to the most noble and their potential voltage (note: the table is not complete). Corrosion Potential in mV Magnesium and Magnesium Alloys -1600 to -1630 Zinc: -980 to -1030 Aluminum: - 760 to -1000 Mild Steel: -600 to 710 Copper; -300 to -570 Brass: -300 to -400 Lead: -190 to -250 18-8 S/S Type 304: -50 to -100 18-8 S/S Type 316: 0 to -100 Graphite: +200 to +300 In order to have proper protection, each metal in the grounding circuit should have a reading at least -200 mV below its stated potential range of corrosion. Example: a brass through hull should give a reading of no more than -500 to -600mV (-300 + -200 = - 500 mV) to have proper protection in the grounding system. Another way of looking at it is that the zinc should be absorbing the electrolysis given its corrosion potential at -980 to -1030 and will function as the anodic agent in the current flow between all the equipment connected in that series of grounding. If the voltage at the brass through-hull is -300 to -400, that means that it is functioning as a potential "sacrificial" anode. The voltage on all underwater hardware connected to the bonding system should be the same (IF NOT, there are problems in the wiring or connections)
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Depends on your equipment

If you have all plastic thru-hulls and only a SS prop shaft and bronze prop then you have a big problem. If you have a lot of metal thru-hulls and bond them together you could just be experiencing normal zinc usage. WM has a great article on setting up your boat to protect you from corrosion. The basic tenanent being disconnect everything electrically, use plastic as much as possable and use zinc as a last resort. This went a long way to reducing my zinc usage (made my radio work better to boot). We went from 2 prop shaft zincs per year to one every other year. And that was in a crowded marina.
 
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